How to Calculate Heat Index
What is Heat Index?
The heat index (apparent temperature or "feels like" temperature) combines air temperature and relative humidity to show how hot it actually feels, since humidity impairs the body's ability to cool through sweating.
Formula
HI ≈ −42.379 + 2.04901523×T + 10.14333127×RH − 0.22475541×T×RH (°F, %)
- HI
- Heat Index (°F or °C)
- T
- Air temperature (°F)
- RH
- Relative humidity (%)
Step-by-Step Guide
- 1Heat index = Rothfusz regression equation with 9 terms
- 2Simplified: HI ≈ −42.379 + 2.049T + 10.143RH − ... (°F)
- 3Only meaningful when T > 27°C (80°F) and RH > 40%
- 4Heat stroke risk when HI > 40°C (103°F)
Worked Examples
Input
Temperature 35°C, humidity 70%
Result
Heat index ≈ 44°C (feels like 44°C due to high humidity)
Frequently Asked Questions
Why is heat index different from temperature?
Heat index accounts for body's inability to cool via sweat evaporation. High humidity + heat = dangerous.
What heat index is dangerous?
>90°F: heat cramps likely. >103°F: heat exhaustion. >125°F: heat stroke risk high.
Can I use wind chill in summer?
No. Wind chill applies to cold. Wind actually increases heat stress by reducing cooling. Use heat index instead.